1. Field of the Invention
The present invention relates to a solid-state image-sensing device provided with pixels that output electrical signals according to incident light, and more particularly to a solid-state image-sensing device whose pixels are each composed of transistors.
2. Description of Related Art
Finding many uses, solid-state image-sensing devices are largely classified into a CCD type and a CMOS type according to the means they use to read photoelectric charges produced by photoelectric conversion elements. In a CCD-type solid-state image-sensing device, photoelectric charges are transferred while being accumulated in potential wells. This leads to a disadvantageously narrow dynamic range. On the other hand, in a CMOS-type solid-state image-sensing device, photoelectric charges accumulated in the pn-junction capacitance of photodiodes are directly read out via MOS transistors.
Some conventional CMOS-type solid-state image-sensing devices operate logarithmically by logarithmically converting the amount of incident light (see JP-A-H11-313257). These solid-state image-sensing devices offer wide, namely five- to six-digit, dynamic ranges, and thus permit, even when a subject having a slightly wider-than-usual brightness distribution is shot, all the brightness information within the brightness distribution to be converted into electrical signals for output. This, however, makes the shootable brightness range wider relative to the brightness distribution of the subject, and thus creates, in low-brightness and high-brightness regions within the shootable brightness range, regions where no brightness data is available.
Under this background, the applicant of the present invention once proposed a CMOS-type solid-state image-sensing device that can be switched between linear and logarithmic conversion as described above (see JP-A-2002-077733). The applicant of the present invention also once proposed a CMOS-type solid-state image-sensing device in which, for the purpose of automatically performing such switching between linear and logarithmic conversion, the transistors connected to photodiodes that perform photoelectric conversion are brought into a proper potential state (see JP-A-2002-300476). In this solid-state image-sensing device, by changing the potential state of those transistors, the inflection point across which their photoelectric conversion switches from linear to logarithmic conversion or vice versa can be changed.
Some other conventional solid-state image-sensing devices are provided with pixels employing a buried photodiode PD as shown in FIG. 11. The pixel shown in FIG. 11 is provided with: a buried photodiode PD formed by forming a P-type well layer 21 on a P-type substrate 20, then forming a P-type layer 10 in the surface of the P-type well layer 21 so as to bury an N-type buried layer 11; a transfer gate TG formed by forming, on the surface of a region adjacent to the region where the buried photodiode PD is formed, a gate electrode 13 with an insulating film 12 laid in between; and an N-type floating diffusion region FD formed in a region adjacent to the region where the transfer gate TG is formed.
In the thus structured pixel shown in FIG. 11, the gate voltage at the gate electrode 13 determines the potential state at the transfer gate TG, and, by using this voltage as the inflection point voltage, it is possible to switch between linear conversion, whereby an electrical signal is produced that varies linearly with respect to the amount of incident light, and logarithmical conversion, whereby an electrical signal is produced that varies logarithmically with respect to the amount of incident light. Here, the potential states of the buried photodiode PD, the transfer gate TG, and the N-type floating diffusion region FD in the pixel have a relationship as shown in FIG. 12A.
When the buried photodiode PD receives light, it produces a photoelectric charge. As a result, according to the produced photoelectric charge, the potential at the buried photodiode PD rises. Here, when the brightness of the subject is low, the charge at the buried photodiode PD is linearly proportional to the integral of the amount of incident light. On the other hand, when the brightness of the subject is high, as the potential at the buried photodiode PD rises and its difference from the potential at the transfer gate TG becomes close to a threshold level, the transfer gate TG starts to operate in a subthreshold region, causing a current to flow out of the buried photodiode PD. Now, as shown in FIG. 12A, the potential at the buried photodiode PD so varies as to be proportional to the logarithm of the current produced by photoelectric conversion.
After the potential at the buried photodiode PD has changed in this way according to the amount of incident light, the gate voltage of the gate electrode 13 is lowered to raise the potential at the transfer gate TG as shown in FIG. 12B. In this state, the charge at the buried photodiode PD is held as shown in FIG. 12B. Subsequently, the charge held in the buried photodiode PD is transferred via the transfer gate TG to the N-type floating diffusion region FD, and an electrical signal based on the thus transferred charge is outputted as an image signal.
In the solid-state image-sensing device operating as described above, global exposure, whereby image sensing is performed simultaneously in all the pixels, is achieved by global shuttering or global resetting. In global shuttering, the accumulation of an electric charge in the buried photodiode PD as described above is performed for the same period in all the pixels, and then, with the same timing, the potential at the transfer gate TG is lowered so that the photoelectric charge accumulated in the buried photodiode PD is transferred to the N-type floating diffusion region FD. After the photoelectric charge obtained by performing image sensing in all the pixels is transferred pixel by pixel to the N-type floating diffusion region FD in this way, an image signal commensurate with the thus transferred photoelectric charge is outputted pixel by pixel.
On the other hand, in global resetting, a mechanical shutter is used as mechanically light-shielding means; that is, the period for which all the pixels are exposed to light is determined by the period for which the mechanical shutter is kept open. When the mechanical shutter is closed so that practically no more photoelectric charge is accumulated in the buried photodiode PD, the N-type floating diffusion region FD is reset pixel by pixel so that a noise signal commensurate with the reset level of the N-type floating diffusion region FD is outputted. Subsequently, via the transfer gate TG, the photoelectric charge accumulated in the buried photodiode PD is transferred to the N-type floating diffusion region FD, and then an image signal commensurate with the photoelectric charge accumulated in the pixel during image sensing is outputted pixel by pixel.
Certainly, global shuttering does not require the provision of mechanically light-shielding means as required by global resetting. In global shuttering, however, the noise signal commensurate with the reset level obtained when the N-type floating diffusion region FD is reset is read out after the image signal is read out, and this makes it impossible to completely eliminate kTC noise contained in the image signal. By contrast, in global resetting, first the noise signal is outputted and then the photoelectric charge is transferred to the N-type floating diffusion region FD so that the image signal is outputted, and thus, certainly, global resetting permits elimination of kTC noise. Global resetting, however, requires extra provision of mechanically light-shielding means, such as a mechanical shutter.
Moreover, in a solid-state image-sensing device provided with pixels that perform logarithmic conversion, if it is not provided with integrating circuits, no electric charge can be accumulated, and therefore the amount of electric charge commensurate with the amount of light at the very moment that the photoelectric charge is held is held in the buried photodiode PD. Thus, if global resetting is used to eliminate kTC noise, a discrepancy between the timing with which the mechanically light-shielding means shields light and the timing with which the photoelectric charge is held in the buried photodiode PD makes it impossible to hold the photoelectric charge converted logarithmically with respect to the amount of incident light, and thus makes it impossible to read out an image signal commensurate with the amount of incident light during image sensing.